Frequently a physician will prescribe two or more medications to be taken by a patient either simultaneously or as close to simultaneously as practical. This is often the case in prescribing insulin. Two basic types of insulin are most frequently prescribed: regular, a fast-acting variety, and NPH, an intermediate-acting variety. Additionally, a longer-acting type of insulin called ultralente is also available. These types of insulin differ in their onset and duration of action. Regular insulin typically has the quickest onset and the briefest duration of action. The onset and duration of insulin may be modified by chemical manipulations which include precipitation with protamine or suspension with isophane or zinc. Injectable insulin is derived from pigs and cattle, and a semi-synthetic genetically-engineered human form is available commercially.
Regardless of the final chemical composition or origin of the insulin prescribed, the physician frequently advises the patient to mix two or more types of insulin and to inject them at least once a day. Some patients respond best to a schedule of two or three daily doses, usually in conjunction with meals. Frequently trial-and-error type of evaluation is practiced to arrive at the correct dosing and combination for the individual patient. However, after this initial adjustment period, the patient may be maintained on the same dose and proportion of insulins for many weeks or months assuming that the patient's diet, activity, and state of health remain reasonably constant or predictable. An example of a typical mixture and ratio of insulins is 70% NPH and 30% regular insulin injected just prior to a meal.
The implications of insulin therapy for the patient generally include the need for two separate sets of insulin injection syringes and needles. This is because the patient will usually draw a predetermined amount of insulin from each of two vials. A sophisticated patient may be able to use a single syringe and simply draw up the correct amount from each separate insulin vial. However, he may prefer to use two separate needles because of the danger of mixing two types of insulin by using a needle "contaminated" with one type of insulin and accidentally getting some of the first insulin into the second vial from which he draws the second insulin. If the patient is not sophisticated or not adept at drawing up insulin from a vial into a syringe, he would need two needles and two syringes. He may find that he has drawn up too much insulin and then he wishes to return the excess to the vial. If he is using a single syringe, he cannot return the excess to the vial if he is presently attempting to draw insulin from the second vial. The problem of getting the correct amount and proportion of each type of insulin prescribed is an everyday difficulty which the diabetic patient may face two or three times a day.
Besides insulin, other medications may be advantageously prescribed as a simultaneous or near-simultaneous dose. For example, pain medications of the opiate family are frequently prescribed together with antiemetics. The antiemetic drugs are often useful as potentiators of narcotic painkillers and additionally they may ameliorate some of the side effects of narcotics such as nausea and vomiting. For example, a patient who has chronic pain, such as that associated with metastatic cancer, may take 50 milligrams of meperidine (a narcotic) and 25 milligrams of hydroxyzine (an antiemetic) intramuscularly several times a day. Patients having chronic pain are often managed in a long-term care facility such as a nursing home. Alternatively, an outpatient setting is sometimes feasible if the patient or his family can administer medications.
Unfortunately, many patients requiring either chronic pain medication or insulin find that they have some difficulty with their eyesight. This may be especially true of the older age group. Additionally, diabetics who require insulin replacement frequently suffer deterioration of vision as a consequence of the diabetic process. Thus, people who are often in greatest need of multiple injected medications of a defined dose and proportion may ironically find their drug regimen most difficult to regulate precisely. Another problem arises in the context of long-term care facilities such as nursing homes. Frequently the nurse who administers injectable drugs is pressed for time on medication rounds. The time pressure is compounded if the care facility is constrained by budgetary or other non-medical concerns. A device which permits accurate dosing and proportion of two or more medications and which allows for repetitive dosing in the same proportions could be useful to a variety of patients in a variety of circumstances.